Pressure wave generator and controller for generating a pressure wave in a fusion reactor

ABSTRACT

An apparatus for generating a pressure wave for activating a fusion reaction in fusionable material in a liquid medium is disclosed. The apparatus includes a plurality of pressure wave generators having respective moveable pistons, the pistons having respective control rods connected thereto. The apparatus also includes a plurality of transducers coupled to the liquid medium and means for causing the pistons of respective ones of the plurality of the pressure wave generators to be accelerated toward respective ones of the plurality of transducers. The apparatus further includes means for causing restraining forces to be applied to respective control rods to cause respective pistons to impact respective transducers at respective desired times and with respective desired amounts of kinetic energy such that the respective desired amounts of kinetic energy are converted into a pressure wave that converges toward the fusionable material in the liquid medium.

This application is related to the U.S. patent application entitled“Magnetized Plasma Fusion Reactor” by Laberge, filed concurrentlyherewith and incorporated herein by reference and to the U.S. patentapplication entitled “Fusionable Material Target”, by Laberge filedconcurrently herewith and incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to nuclear fusion reactors and more particularlyto pressure Wave generation in nuclear fusion reactors.

2. Description of Related Art

Nuclear fusion reactions involve bringing together atomic nuclei againsttheir mutual electrostatic repulsion and fusing them together to makeheavier nuclei, while at the same time releasing energy. Isotopes oflight elements (i.e. elements having a relatively small number ofprotons) are the easiest to fuse, because the electrostatic repulsionbetween the nuclei of light elements is smaller than that of heavierelements. The use of light elements may produce significantly reducedcollateral radioactivity than comparable fission reactors, whichtypically use isotopes of heavier elements.

Inducing nuclear fusion reactions is difficult, because of the energiesrequired to accelerate the nuclei to speeds fast enough to overcometheir mutual electrostatic repulsion and because the nuclei are so smallthat the chance that two passing nuclei will interact with one anotherin a manner which results in fusion of the nuclei is small.

Fusion reactors typically require input energy to initiate fusionreactions. The amount of input energy required is largely determined bythe need to accelerate the nuclear reactants to thermonuclear speed andto confine the nuclear reactants in a space that allows them tointeract. A reactor that consumes less energy than it produces is saidto produce net energy. Such a reactor will have an efficiency ratio (theratio of energy output to the energy input) greater that unity. Theenergy output of a fusion reactor is largely determined by the number offusion reactions that are induced in the reactor and the amount ofenergy that is released and captured.

There remains a need for methods and apparatus that facilitateimprovements to the efficiency of nuclear fusion reactors.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention there is provided amethod of operating a pressure wave generator in a system of pressurewave generators for generating a pressure wave in a liquid mediumcontained in a fusion reactor, wherein each pressure wave generator hasa moveable piston and a control rod coupled thereto. The method includescausing the piston to be accelerated toward a transducer coupled to theliquid medium, by applying a motive force to the piston. The methodfurther includes applying a restraining force to the control rod tocause the piston to impact the transducer at a desired time and with adesired kinetic energy such that the kinetic energy is converted into apressure wave in the liquid medium.

Applying the motive force may involve applying a fluid pressure to thepiston.

Causing the piston to be accelerated may involve applying a holdingforce to the control rod operable to hold the piston stationary whileapplying a fluid pressure to the piston.

The method may involve using a brake to apply the holding force.

Causing the piston to be accelerated may involve releasing a latchcoupled to at least one of the control rod and the piston, the latchbeing operable to hold the piston stationary while applying a fluidpressure to the piston.

The method may include generating a position signal representing aposition of the piston and applying the restraining force in response tothe position signal.

Generating the position signal may involve generating a signalrepresenting a position of the control rod.

Applying the restraining force may involve applying the restrainingforce in response to differences between positions of the piston anddesired piston positions from a schedule of positions representing adesired piston position relative to time.

Applying the restraining force may involve increasing the restrainingforce when a position of the piston is ahead of a scheduled position anddecreasing the restraining force when the position of the piston isbehind the scheduled position.

Applying the restraining force may involve producing a restraining forcein response to applying a transfer function to at least one of thedifferences.

The method may involve modifying the transfer function in response to atleast one of the differences, such that respective differences in asubsequent operation of the piston are minimized.

In accordance with another aspect of the invention there is provided apressure wave generator apparatus for use in a system of pressure wavegenerators for generating a pressure wave in a liquid medium containedin a fusion reactor. The apparatus includes a moveable piston, a controlrod coupled to the piston, and a transducer coupled to the liquidmedium. The apparatus further includes provisions for causing the pistonto be accelerated toward the transducer, by causing a motive force to beapplied to the piston and provisions for causing a restraining force tobe applied the control rod to cause the piston to impact the transducerat a desired time and with a desired kinetic energy such that thekinetic energy is converted into a pressure wave in the liquid medium.

The provisions for causing the motive force to be applied may includeprovisions for applying a fluid pressure to the piston.

The apparatus may include provisions for causing a holding force to beapplied to the control rod, the holding force operable to hold thepiston stationary while applying a fluid pressure to the piston.

The apparatus may include provisions for generating a position signalrepresenting a position of the piston.

The provisions for causing the restraining force to be applied to thecontrol rod may be operably configured to cause the restraining force tobe applied in response to the position signal.

The provisions for generating the position signal may include provisionsfor generating a signal representing a position of the control rod.

The provisions for causing the piston to be accelerated toward thetransducer may include provisions for directing the piston toward a wallcontaining the liquid medium in the fusion reactor such that the pistonimpacts the wall and wherein the wall acts as the transducer coupled tothe liquid medium, such that the impact of the piston against the wallcauses a pressure wave to be generated in the liquid medium.

The apparatus may include provisions for guiding the piston toward thetransducer.

The provisions for guiding may include provisions for at least partiallyevacuating air from movement path of the piston.

The provisions for guiding the piston may include a housing having aninside bore.

The apparatus may include provisions for generating an air cushionbetween the piston and the inside bore operable to reduce frictionalforces between the piston and the bore.

The transducer may include provisions for impedance matching thetransducer to the liquid medium.

The piston may include a face operable to impact a face of thetransducer and the transducer may include provisions for reducinglocalized impact stresses between the face of the piston and the face ofthe transducer.

In accordance with another aspect of the invention there is provided apressure wave generator apparatus for use in a system of pressure wavegenerators for generating a pressure wave in a liquid medium containedin a fusion reactor. The pressure wave generator apparatus includes amoveable piston, a control rod coupled to the piston and a transducercoupled to the liquid medium. The apparatus further includes a motiveforce generator for causing the piston to be accelerated toward thetransducer and a brake for causing a restraining force to be applied thecontrol rod to cause the piston to impact the transducer at a desiredtime and with a desired kinetic energy such that the kinetic energy isconverted into a pressure wave in the liquid medium.

The motive force generator may include a housing for guiding the piston,the housing defining a first cavity behind the piston, the cavity havinga fluid port for applying a fluid pressure to the cavity operable toaccelerate the piston toward the transducer.

The housing may define a second cavity in front of the piston and mayinclude a vacuum port in the second cavity operable to facilitate theacceleration of the piston by at least partially evacuating the secondcavity.

The apparatus may include a brake for causing a holding force to beapplied to the control rod, the holding force operable to hold thepiston stationary while applying a fluid pressure to the piston.

The apparatus may include a latch coupled to at least one of the controlrod and the piston, the latch being operably configured to be releasedwhile fluid pressure is applied to the piston to permit the piston toaccelerate under the fluid pressure.

The apparatus may include a position sensor for generating a positionsignal representing a position of the piston.

The control rod may include a plurality of indicia on a surface thereofand the position sensor may include an illuminator for directing a beamof light towards the indicia and a photodetector for generating a signalrepresenting an intensity of light reflected from the indicia, such thatwhen the piston is accelerated, movement of the control rod causes thephotodetector to generate a signal of varying intensity representing theposition of the control rod.

The brake may be operably configured to cause the restraining force tobe applied in response to the position signal.

The brake may be operably configured to cause the restraining force tobe applied in response to differences between positions of the pistonand desired piston positions from a schedule of positions representingdesired piston positions relative to time.

The brake may be operably configured to cause the restraining force tobe increased when a position of the piston is ahead of a scheduledposition and to cause the restraining force to be decreased when theposition of the piston is behind the scheduled position.

The brake may be operably configured to cause the restraining force tobe applied in response to applying a transfer function to thedifferences.

The apparatus may include a controller for modifying the transferfunction in response to the differences, such that respectivedifferences in a subsequent operation of the piston are minimized.

The motive force generator may be operably configured to direct thepiston toward a wall containing the liquid medium in the fusion reactorsuch that the piston impacts the wall and wherein the wall acts as thetransducer coupled to the liquid medium, such that the impact of thepiston against the wall causes a pressure wave to be generated in theliquid medium.

The transducer may include a member mounted on a wall containing theliquid medium in the fusion reactor and wherein the pressure wavegenerator apparatus is coupled to the wall such that the piston isdisposed to impact the member.

The apparatus may include a housing for guiding the moveable piston, thehousing having an outside surface and an inside bore.

The outside surface may be operable to fit complementarily into anopening in a wall containing the liquid medium in the fusion reactor.

The piston may include a plurality of fluid orifices disposed betweenthe piston and the inside bore of the housing, the orifices beingoperably configured receive pressurized fluid and to generate an aircushion between the piston and the inside bore for reducing frictionalforces between the piston and the bore.

The brake may include an actuator and a brake pad operable to generatethe restraining force by frictionally engaging a surface of the controlrod in response to an actuation force applied by the actuator.

The actuator may include a piezoelectric material.

The brake may include a magnetic circuit operably configured toestablish a magnetic field through the control rod thereby generatingeddy currents in the control rod when the control rod moves with respectto the magnetic circuit, the generation of the eddy currents operable toapply the restraining force to the control rod.

The brake may include a magnetic fluid in contact with the control rodand a magnetic circuit operably configured to generate the restrainingforce by causing a magnetic field to be coupled through the magneticfluid and the control rod.

In accordance with another aspect of the invention there is provided amethod of generating a pressure wave for activating a fusion reaction infusionable material in a liquid medium. The method involves causingpistons of respective ones of a plurality of pressure wave generators tobe accelerated toward respective transducers coupled to the liquidmedium, by applying respective motive forces to the pistons. The methodalso involves causing restraining forces to be applied to respectivecontrol rods connected to respective pistons to cause the respectivepistons to impact the transducer at respective desired times and withrespective desired amounts of kinetic energy such that the respectivedesired amounts of kinetic energy are converted into a pressure wavethat converges toward the fusionable material in the liquid medium.

The method may include introducing fusionable material into the liquidmedium.

The method may include locating the fusionable material in the liquidmedium.

The method may include determining the desired times and the desiredamounts of kinetic energy in response to a location of the fusionablematerial.

The method may include producing location signals representing alocation of the fusionable material in the liquid medium.

The method may include producing release signals for causing the pistonsto be accelerated and producing restraining signals for causing therestraining force to be applied to the control rods in response to thelocation signals.

The method may include receiving the release signals at actuators andcausing the actuators to release the pistons for movement in response tothe release signals.

The method may include receiving the restraining signals at brakes andcausing the brakes to apply the restraining forces to the control rodsin response to the restraining signals.

At least one of the desired times and desired kinetic energies may bedetermined in response to the location signals.

The method may include generating position signals representingpositions of respective pistons and causing the restraining forces to beapplied in response to the position signals and the location signals.

In accordance with another aspect of the invention there is provided acomputer readable medium encoded with codes for directing a processorcircuit to carry out the method and any of its variations above.

In accordance with another aspect of the invention there is provided acomputer readable signal encoded with codes for directing a processorcircuit to carry out the method and any of its variations above.

In accordance with another aspect of the invention there is provided anapparatus for generating a pressure wave for activating a fusionreaction in fusionable material in a liquid medium. The apparatusincludes a plurality of pressure wave generators having respectivemoveable pistons, the pistons having respective control rods connectedthereto. The apparatus also includes a plurality of transducers coupledto the liquid medium and provisions for causing the pistons ofrespective ones of the plurality of the pressure wave generators to beaccelerated toward respective ones of the plurality of transducers. Theapparatus further includes provisions for causing restraining forces tobe applied to respective control rods to cause respective pistons toimpact respective transducers at respective desired times and withrespective desired amounts of kinetic energy such that the respectivedesired amounts of kinetic energy are converted into a pressure wavethat converges toward the fusionable material in the liquid medium.

The apparatus may include provisions for causing fusionable material tobe introduced into the liquid medium.

The apparatus may include provisions for locating the fusionablematerial in the liquid medium.

The apparatus may include provisions for determining the desired timesand the desired amounts of kinetic energy in response to a location ofthe fusionable material.

The apparatus may include provisions for producing location signalsrepresenting a location of the fusionable material in the liquid medium.

The provisions for causing the pistons to be accelerated may includemotive force generating provisions for generating forces on the pistonsin respective directions of desired movement of the respective pistonsand holding provisions for holding the piston stationary while theforces are applied.

The apparatus may include provisions for producing release signalsoperable to be received by the holding provisions and the holdingprovisions being responsive to the release signals to release thepistons to cause the pistons to be accelerated in response to the forcesgenerated by the motive force generating provisions and the apparatusmay further include provisions for producing restraining signals tocause the restraining force to be applied to the control rods inresponse to the location signals.

The apparatus may include provisions for generating position signalsrepresenting positions of respective pistons to cause the restrainingforces to be applied in response to the position signals and thelocation signals.

The provisions for causing restraining forces to be applied may beoperably configured to determine at least one of the desired times anddesired kinetic energies in response to the location signals.

In accordance with another aspect of the invention there is provided anapparatus for generating a pressure wave for activating a fusionreaction in fusionable material in a liquid medium. The apparatusincludes a plurality of pressure wave generators having respectivemoveable pistons, the pistons having respective control rods connectedthereto. The apparatus further includes a plurality of transducerscoupled to the liquid medium and a plurality of motive force generatorsfor causing the pistons of respective ones of the plurality of thepressure wave generators to be accelerated toward respective ones of theplurality of transducers. The apparatus also includes a plurality ofbrakes for causing restraining forces to be applied to respectivecontrol rods to cause respective pistons to impact respectivetransducers at respective desired times and with respective desiredamounts of kinetic energy such that the respective desired amounts ofkinetic energy are converted into a pressure wave that converges towardthe fusionable material in the liquid medium.

The apparatus may include an aperture for causing fusionable material tobe introduced into the liquid medium.

The apparatus may include a fusionable material locating system operableto locate the fusionable material in the liquid medium.

The apparatus may include a controller for determining the desired timesand the desired amounts of kinetic energy in response to a location ofthe fusionable material.

The apparatus may include location sensors for producing locationsignals representing a location of the fusionable material in the liquidmedium.

The motive force generators may be operably configured to generateforces on the pistons in respective directions of desired movement ofthe respective pistons and the apparatus may include a brake for holdingthe piston stationary while the forces are applied.

The apparatus may include a controller for producing release signalsoperable to be received by the brakes, the brakes being responsive tothe release signals to release the pistons to cause the pistons to beaccelerated in response to the forces generated by the motive forcegenerators the brakes operable configured for producing restrainingsignals to cause the restraining force to be applied to the control rodsin response to the location signals.

The apparatus may include position sensors for generating positionsignals representing positions of respective pistons to cause therestraining forces to be applied in response to the position signals andthe location signals.

The controller for causing restraining forces to be applied may beoperably configured to determine at least one of the desired times anddesired kinetic energies in response to the location signals.

The apparatus may include a plurality of location sensors operablyconfigured to produce ultrasonic beams and to receive reflections of theultrasonic beams, the reflections of the ultrasonic beams representingan alignment of respective ones of the pressure wave generators.

In accordance with another aspect of the invention there is provided amethod of operating a pressure wave generator in a system of pressurewave generators for generating a pressure wave in a liquid mediumcontained in a fusion reactor. The method includes causing a movingpiston having kinetic energy to impact a moveable transducer coupled tothe liquid medium and converting at least a portion of the kineticenergy into a pressure wave in the liquid medium such that said pressurewave envelopes and converges on a fusionable material in the liquidmedium.

In accordance with another aspect of the invention there is provided apressure wave generator apparatus for use in a system of pressure wavegenerators for generating a pressure wave in a liquid medium containedin a fusion reactor. The apparatus includes a moveable piston, amoveable transducer coupled to the liquid medium and provisions forcausing the piston having kinetic energy to impact the transducer. Theapparatus also includes provisions for converting at least a portion ofthe kinetic energy into a pressure wave in the liquid medium, thepressure wave operable to envelope and converge on a fusionable materialin the liquid medium.

The apparatus may include provisions for guiding the piston toward thetransducer.

The transducer may include provisions for impedance matching thetransducer to the liquid medium.

The piston may include a face operable to impact a face of thetransducer and wherein the transducer may include provisions forreducing localized impact stresses between the face of the piston andthe face of the transducer.

In accordance with another aspect of the invention there is provided apressure wave generator apparatus for use in a system of pressure wavegenerators for generating a pressure wave in a liquid medium containedin a fusion reactor. The apparatus includes a moveable piston, amoveable transducer coupled to the liquid medium and a motive forcegenerator for causing the piston having kinetic energy to impact thetransducer such that at least a portion of the kinetic energy isconverted into a pressure wave in the liquid medium, the pressure Wavebeing formed such that it envelopes and converges on a fusionablematerial in the liquid medium.

The transducer may include a plurality of layers of materials havingtransmission properties, each material having different transmissionproperties, the materials being selected and arranged in the layers suchthat the transducer is generally impedance matched to the liquid medium.

The transducer may include a member mounted on a wall containing theliquid medium in the fusion reactor and wherein the pressure wavegenerator apparatus is coupled to the wall such that the piston isdisposed to impact the member.

The apparatus may include a housing having an outside surface and aninside bore, the inside bore operable to guide the piston toward thetransducer.

The outside surface may be operable to fit complementarily into anopening in a wall containing the liquid medium in the fusion reactor.

The housing may include a first area defined by a first wall portionoperably configured to hold the transducer in a position in which itwill be impacted by the piston.

The transducer may include a member having an outside surface having afirst portion defining a shape complementary to the first wall portion.

The housing may include a first area defined by a first wall portionhaving a first inside diameter and wherein the housing has a second areadefined by a second wall portion having a second inside diameter andwherein the housing has a third area defined by a tapered third wallportion located between the first and second wall portions, the firstwall portion being operable to guide the piston and the second and thirdwall portions being operable to hold the transducer.

The transducer may include a member having an outside surface havingfirst and second portions defining a shape complementary to the secondand third wall portions of the housing.

The first inside diameter may be less than the second inside diameter.

The piston may include a face operable to impact a face of thetransducer and the transducer may include a conformal member forreducing localized impact stresses between the face of the piston andthe face of the transducer.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings which illustrate embodiments of the invention,

FIG. 1 is a perspective view of a fusion reactor according to a firstembodiment of the invention;

FIG. 2 is a perspective view of a portion of the fusion reactor shown inFIG. 1;

FIG. 3 is a cross-sectional view of the fusion reactor shown in FIG. 1taken along line 3-3;

FIG. 4 is a perspective view of a pressure wave generator used in thefusion reactor shown in FIG. 1;

FIG. 5 is a cross-sectional view of the pressure wave generator shown inFIG. 4;

FIG. 6 is a block diagram of a processor circuit for implementing acontroller for the pressure wave generator shown in FIG. 4;

FIG. 7 is a flowchart of codes executed by the processor circuit of FIG.6 to implement a controller for initializing the pressure wave generatorshown in FIG. 4;

FIG. 8 is a flowchart of codes executed by the processor circuit of FIG.6 to implement a controller for firing the pressure wave generator shownin FIG. 4;

FIG. 9 is a cross-sectional view of an embodiment of a brake forimplementing the pressure wave generator of FIG. 4;

FIG. 10 is a plane view of the brake shown in FIG. 9;

FIG. 11 is a cross-sectional view of another embodiment of a brake forimplementing the pressure wave generator of FIG. 4;

FIG. 12 is a plane view the brake shown in FIG. 11;

FIG. 13 is a cross-sectional view of yet another embodiment of a brakefor implementing the pressure wave generator of FIG. 4;

FIG. 14 is a plane view the brake shown in FIG. 13;

FIG. 15 is a schematic view of a position sensor for implementing thepressure wave generator of FIG. 4;

FIG. 16 is a plane view of a reticule of the position sensor shown inFIG. 15;

FIG. 17 is a schematic representation of signal waveforms produced bythe position sensor of FIG. 15;

FIG. 18 is a perspective view of a system for fabricating a control rodfor implementing the pressure wave generator of FIG. 4;

FIG. 19 is a cross-sectional view of the control rod fabricated by thesystem shown in FIG. 18;

FIG. 20 is a detailed cross-sectional view of a portion of the pressurewave generator shown in FIG. 4;

FIG. 21 is a cross-sectional view of an alternative embodiment of apressure wave generator; and

FIG. 22 is a cross-sectional view of another alternative embodiment of apressure wave generator.

DETAILED DESCRIPTION

Commonly owned U.S. patent application Ser. No. 10/507,323, filed onMar. 12, 2002 is incorporated herein by reference and describes theconstruction and operation of a fusion reactor.

Referring to FIG. 1, a fusion reactor according to a first embodiment ofthe invention is shown generally at 100. The fusion reactor 100 includesa wall 102 and a plurality of radially oriented pressure wave generators104, symmetrically arranged around an exterior of the wall. Referring toFIG. 2, in one embodiment, the wall 102 of the fusion reactor 100 mayinclude a lower hemispherical shell 300 including a plurality ofopenings 302. The wall 102 of the fusion reactor 100 may also include acomplimentary upper hemispherical shell shown at 203 in FIG. 1.

Referring to FIG. 3, the wall 102 of fusion reactor 100 defines an innercavity 122 for containing a liquid medium 120. The liquid medium 120 maybe a molten metal, such as lead, lithium, or sodium, or an alloy of suchmetals, and may be maintained under pressure (when the liquid is lithiumthe pressure may be in the region of 100 bar). The liquid medium 120 mayalso contain additives that enhance the properties thereof, for exampleby enhancing neutron shielding or increasing the density of the liquidmedium.

The wall 102 of the fusion reactor 100 further includes an inletaperture 124 and an outlet aperture 128 disposed on diametricallyopposite sides of the reactor. The fusion reactor 100 also includes aninlet conduit 125 in communication with the inlet aperture 124 and anoutlet conduit 126 in communication with the outlet aperture 128. Thefusion reactor 100 further includes a recirculation system 130, whichincludes an input 134 in communication with the outlet conduit 126 andan output 132 in communication with the inlet conduit 125. Therecirculation system 130 also includes a pump (not shown) forcirculating the liquid medium 120 through the fusion reactor 100 and mayalso include facilities for maintaining the liquid medium at a desiredtemperature by extracting heat. The recirculation system 130 may alsoinclude a turbine (not shown) for converting the heat into electricalenergy.

The fusion reactor 100 also includes a reservoir 136, in communicationwith the inlet conduit 125, for holding fusionable material 138 and forintroducing the fusionable material 138 into the liquid medium 120through the inlet conduit 125. The fusionable material 138 may be in agaseous form and may include an isotope of a light element, such asdeuterium, tritium, 3He, or a combination thereof. The fusionablematerial 138 may also include an encapsulating wall which may includeglass, plastic or other suitable materials.

The fusion reactor 100 further includes a controller 142, which mayinclude a locating system not shown) for locating a fusionable materialtarget 140 within the liquid medium 120. The locating system includes aplurality of position sensors 152 located on the wall 102 of the fusionreactor 100. The position sensors 152 may include ultrasonictransceivers having inputs for receiving an excitation pulse that causesthe ultrasonic transceiver to transmit an ultrasonic pulse that couplesthrough the wall 102 into the liquid medium 120. The ultrasonictransceivers may also include outputs that produce location signals inresponse to a received reflection from a fusionable material target 140.The controller 142 includes a plurality of inputs 154 for receivinglocation signals from the transceivers.

The controller 142 further includes a plurality of inputs 148, coupledto respective outputs 150 of the pressure wave generators 104 and aplurality of outputs 144 coupled to respective inputs 146 of thepressure wave generators 104. The controller 142 is described in greaterdetail below.

The pressure wave generators 104 include a moveable piston 410, atransducer 412 and a motive force generator 112 for accelerating thepiston from an initial position 110 (shown in broken outline) to impactthe transducer 412. The motive force generator 112 may include acylinder and a fluid inlet 114 for applying a fluid pressure to thepiston to generate the motive force. In this embodiment the transducer412 is slideably received in an opening 302 in the wall 102 and iscapable of being displaced radially relative to the wall 102 by theimpact of the piston 410. In other embodiments the transducer includes aportion of the wall 102 that may be directly impacted by the piston 410.

Referring to FIG. 4, the pressure wave generator 104 is shown in greaterdetail. The pressure wave generator 104 includes a housing 400 and anend cap 402. The end cap 402 includes a fluid port 408 for applyingfluid pressure to the pressure wave generator 104. The housing 400includes an outside surface 414 that is dimensioned to fitcomplementarily in one of the plurality of openings 302 (shown in FIG.2). The pressure wave generator 104 further includes a flange 404 and aplurality of clamps 406 for mounting the pressure wave generator on thewall 102 of the fusion reactor 100.

Referring to FIG. 5, the pressure wave generator 104 is shown in asectional view. The housing 400 of the pressure wave generator 104includes an inside bore 418 for accommodating the moveable piston 410.The housing 400 further includes a wall portion 420 for holding atransducer 412. The transducer 412 is in contact with and coupled to theliquid medium. 120, thus facilitating the exchange of energy between thetransducer and the liquid medium.

Referring to FIG. 20, the piston 410, the transducer 412, and thehousing 400 of the pressure wave generator 104 are shown in greaterdetail at 2001. The housing 400 includes annular guides 2004 and 2006for locating the pressure wave generator 104 in one of the openings 302in the wall 102 (shown in FIG. 2) of the fusion reactor 100. The housing400 also includes a metal seal 2005 for ensuring that the liquid medium120 does not leak out of the vessel around the pressure wave generator104. The pressure wave generator 104 includes a fastener 2008 forclamping the pressure wave generator to the wall 102 using the clamps406 and the flange 404. The positioning of the pressure wave generator104 within the opening 302 may be adjusted by introducing shims 2012between the flange 404 and the vessel wall 102. The shims 2012 may beused to adjust the longitudinal position of the pressure wave generator104 in the opening 302 as well as to aim the pressure wave generatortoward the center of the inner cavity 122.

The piston 410 (shown near the end of its travel along the bore 418)includes a conduit 2014 in communication with a cavity 546 and a pair oforifices 2016 and 2018, located between the bore 418 and the piston 410.The piston 410 includes a plurality of such conduits (only one shown)located around the circumference of the piston 410. The piston 410 alsoincludes an impact surface 554 and a rear surface 555.

The wall portion 420 of the housing 400 includes a tapered wall portion2022 and bore 2024. The transducer 412 is complementarily shaped to beslideably accommodated in the wall portion 420 and includes an annularguide 2026 for contacting the bore 2024 and providing a sliding fittherewith. The wall portion 420 of the housing 400 also includes aprotruding lip 2028 for preventing the transducer 412 from becomingdislodged from the housing. The transducer 412 also includes an impactsurface 2031 and an outer surface 2030.

Returning again to FIG. 5, the piston 410 includes a control rod 422coupled to the piston, and projecting rearwardly through the end cap402. The control rod 422 includes indicia comprising a plurality ofregularly spaced marks 424, inscribed on a surface of the control rod.

The housing 400 also includes a vacuum orifice 426 in communication witha vacuum conduit 428. The vacuum conduit 428 is connected to a vacuumpump 430 through a vacuum control valve 432. The vacuum control valve432 is electrically actuated by a vacuum control signal coupled to thevalve by the signal line 436.

The pressure wave generator 104 also includes a pressurised fluid supply440, including a compressor 446 for providing pressurised fluid (such ascompressed air), a reservoir 444 for storing pressurised fluid and aregulator 442 for regulating the flow of the pressurised fluid. Theregulator 442 is controlled by a fluid control signal coupled to theregulator by the signal line 448 and facilitates applying and removingfluid pressure to the pressure wave generator 104 through the fluid port408. The regulator 442 further includes facilities for adjusting apressure level of the fluid pressure applied to the pressure wavegenerator 104.

In this embodiment the pressure wave generator 104 also includes a brake434 connected to the end cap 402. The brake 434 is electrically actuatedby a braking control signal coupled to the brake by line 438.

Referring to FIG. 9 and FIG. 10, an embodiment of a brake is showngenerally at 901. The brake 901 includes a brake pad 900, an annularpiezoelectric actuator 902 and a high voltage driver 910. The brake pad900 almost completely encircles the control rod 422 except for a narrowgap 908. The piezoelectric actuator 902 surrounds the brake pad 900. Thepiezoelectric actuator 902 includes electrical input terminals 904 and906. The high voltage driver 910 includes an input terminal 916 forreceiving a braking control signal and a pair of output terminals 912and 914 for producing a high voltage drive signal.

In operation, the high voltage driver 910 receives a braking controlsignal at the input terminal 916 and produces a proportional highvoltage drive signal at the output terminals 912 and 914. Thepiezoelectric actuator 902 applies a radially inwardly directed force tothe brake pad 900 in response to the high voltage drive signal appliedto the input terminals 904 and 906. The radially inwardly directed forcecauses a frictional force to be applied to the control rod 422,restraining, or preventing movement in directions indicated by the arrow918.

Referring to FIG. 11 and FIG. 12, an alternative embodiment of a brakeis shown at 1100. The brake 1100 includes a magnetic core 1108, amagnetic fluid 1104 and a housing 1102 for containing the magneticfluid. A coil 1110 is wound around the magnetic core 1108. The coil 1110includes a pair of terminals 1112 and 1114 for receiving a drivecurrent. The housing 1102 of the brake 1100 is situated in a gap in themagnetic core and includes a seal 1109, which facilitates free movementof the control rod 422 relative thereto, while preventing escape of themagnetic fluid 1104. Magnetic fluid is a composite which includes fineferromagnetic particles, usually dispersed in a base liquid. In theabsence of a magnetic field the fluid flows freely but when a magneticfield is applied the ferromagnetic particles align constraining the freeflow. The brake 1100 further includes a current driver 1116 having aninput terminal 1122 for receiving a braking control signal and havingoutput terminals 1118 and 1120 for delivering a drive current to theterminals 1112 and 1114 of the coil 1110.

In operation, the current driver 1116 receives a braking control signalat the input terminal 1122 and produces a proportional drive currentwhich is applied to the coil 1110 at the terminals 1112 and 1114. Thecurrent supplied to the coil 1110 establishes a magnetic field in amagnetic circuit defined by the magnetic core 1108, the magnetic fluid1104, and the control rod 422. The magnetic field through the magneticfluid 1104 causes the magnetic particles to align thus applying arestraining force to the control rod 422 in directions indicated by thearrow 1124. The amount of restraining force applied is proportional tothe current in the coil 1110.

Another embodiment of a brake is shown in FIG. 13 and FIG. 14 at 1300.The brake 1300 includes a magnetic core 1302 and a coil 1304, woundaround the magnetic core. The brake 1300 also includes an air gap 1310in the magnetic core 1302 and the magnetic core 1302 is positioned suchthat the control rod 422 is moveably positioned in the air gap. The coil1304 includes a pair of terminals 1306 and 1308 for receiving a drivecurrent. The brake 1300 further includes a current driver 1312 having aninput terminal 1318 for receiving a braking control signal and having apair of output terminals 1314 and 1316 for delivering a drive current tothe terminals 1306 and 1308 of the coil 1304.

In operation, the current driver 1312 supplies a current to theterminals 1306 and 1308 of the coil 1304 in response to the brakingcontrol signal received at the input 1318. The current in coil 1304causes a magnetic field to be established in the magnetic core 1302. Themagnetic field couples into the control rod 422, but negligible force isapplied to the control rod while it is stationary. However, when thecontrol rod 422 is moved, the magnetic field generates eddy currents inthe material of the control rod, thus applying a restraining force tothe control rod. The restraining force is proportional to the velocityof the control rod 422 and the current through the coil 1304. In thisembodiment the brake 1300 is unable to apply a holding force to thecontrol rod 422 when the control rod is stationary, requiring anadditional latch to be provided for holding the piston 410 prior tofiring. The latch may include an electromechanical solenoid (not shown)that is electrically activated to remove a plunger holding either thepiston 410 or the control rod 422. Alternatively the holding force maybe applied by a separate brake that applies a holding force to eitherthe piston 410 or the control rod 422.

Returning now to FIG. 5, the pressure wave generator 104 furtherincludes a position sensor 450, located proximate to the control rod422. The position sensor 450 produces a position signal representing theposition of the control rod 422 on the signal line 452.

Referring to FIGS. 15 and 16, an embodiment of the position sensor 450employing optical sensing techniques is shown at 1500. The positionsensor 1500 includes an illuminator 1501, an imaging system 1503, areticule 1512, a first photodetector 1516, and a second photodetector1514. The illuminator 1501 includes a light emitting diode (LED) 1502,an illumination lens 1504 and a beamsplitter 1506. The imaging system1503 includes a first imaging lens 1508 and a second imaging lens 1510.

The reticule 1512 is shown in greater detail in FIG. 16 at 1512. Thereticule 1512 is divided to include a first region 1530 and a secondregion 1532. The regions 1530 and 1532 include a regular pattern oflines defined by a plurality of absorptive lines 1534 and a plurality oftransmissive lines 1536. The lines are spaced apart by a pitch distancep indicated at 1538. The lines in the region 1530 are also offset fromthe lines in the region 1532 by one quarter of the spacing distance p.The first photodetector 1516 is positioned to measure a light intensitytransmitted through the region 1530 of the reticule 1512 and the secondphotodetector 1514 is positioned to measure a light intensitytransmitted through the region 1532 of the reticule 1512. The positionsensor 1500 further includes a signal analyser 1518 having an input 1520coupled to the second photodetector 1514 and an input 1522 coupled tothe first photodetector 1516. The signal analyser 1518 also includes anoutput 1524 for producing a position signal.

The operation of the position sensor 1500 is described with reference toFIGS. 15, 16 and 17. Light Illumination from the LED 1502 is gathered bythe illumination lens 1504 and directed onto the control rod 422 via thebeamsplitter 1506 and the first imaging lens 1508. The beamsplitter 1506is a partially silvered mirror that transmits some portion of the lightand reflects the remaining light (usually 50%). The marks 424 on thecontrol rod 422 reflect the illumination back through the first imaginglens 1508, the beamsplitter 1506, and the second imaging lens 1510 ontothe reticule 1512, forming an image 1526 of the marks 424 at the planeof the reticule. The image 1526 includes a plurality of light areas 1537alternating with a plurality of dark areas 1539, the light and darkareas corresponding to the marks 424 (for clarity the image 1526 isshown separated from the reticule 1512 in FIG. 16, but it should beunderstood that in operation the image impinges on the reticule). Thesecond imaging lens 1510 is selected and positioned such that thespacing between the dark areas 1539 (and the light areas 1537) issubstantially the same as the reticule line spacing p.

The image 1526, impinging on the reticule 1512, generates a Moiréinterference pattern at the reticule i.e. when the light areas 1537 inthe image 1526 line up with the absorptive lines 1534, in the region1532 of the reticule 1512, very little light is transmitted through thereticule to the second photodetector 1514. However, when the control rod422 is displaced in the direction shown by the arrow 1528, the image1526 also moves and when the light areas 1537 in the image 1526 line upwith the transmissive lines 1536 in the region 1532 of the reticule1512, almost all of the light is transmitted through the reticule to thesecond photodetector 1514. For in-between alignments of the image 1526and the lines of the reticule 1512, an attenuated beam of light istransmitted through the reticule. Movement of the control rod 422, thusresults in a time varying light intensity being registered by the secondphotodetector 1514. Similarly movement of the control rod 422, resultsin a time varying light intensity being registered by the firstphotodetector 1516.

Referring to FIG. 17, the signals generated by the photodetectors 1514and 1516 in response to the varying light intensity through the reticule1512 are shown as a pair of periodic signals 1540 and 1542 respectively.The depicted signals 1540 and 1542 represent a specific case where thecontrol rod 422 is moved at a constant velocity, although in practicethe velocity of the control rod will not be constant, resulting in thesignals having varying frequency. The signals 1540 and 1542 are offsetin phase by 90 degrees due to the offset between the lines in the region1530 and the region 1532 on the reticule 1512 (i.e. the signals 1540 and1542 have a quadrature phase relationship). A change in position of thecontrol rod 422 is represented as a change in intensity of the signals1540 and 1542. The velocity of the rod may be calculated from the knownspacing between the marks 424 on the control rod 422 and the timeinterval between a peak 1548 and a peak 1550 of the signal 1540. Eitherone of the signals 1540 or 1542 may be used to represent a change in theposition of the control rod 422. However, either one of the signals 1540and 1542 alone may not unambiguously indicate a direction of motion. Thedirection of motion may be determined by further analysing thequadrature signals 1540 and 1542. The signal analyser 1518 is configuredto interpret the quadrature signals 1540 and 1542 to remove any possibleambiguity regarding the direction of motion. For example, if the controlrod 422 reverses direction at a time indicated by the vertical line1544, the signal 1540 will not indicate this change since the sinusoidalsignal is at its peak 1550 and the intensity of the signal changes itsdirection of swing at this time, whether or not the direction of motionreverses. This creates an ambiguity that may be resolved by examiningthe signal 1542 at the time 1544. If the signal 1542 continues toincrease in intensity past the time 1544, then the direction has notreversed. However should the signal 1542 start to reduce in intensityafter the time 1544 then the direction of motion has unambiguouslyreversed.

In this embodiment, optical sensing techniques are used to generate theposition signal, but other position sensing techniques such as fibreinterferometry may also be employed.

Referring to FIG. 6 the controller 142 for controlling the plurality ofpressure wave generators 104 may be implemented by a processor circuitshown generally at 536. The processor circuit 536 includes a processor602, random access memory (RAM) 604, program memory 606, an input/output(I/O) interface 608, and a media reader 610, all in communication withthe processor 602. The RAM 604 and the program memory 606 may, ofcourse, be integrated within the processor 602 itself. In addition, theI/O interface 608 may also be integrated within the processor 602. TheI/O interface 608 includes an input 544 for receiving a position signal.The I/O interface may include analog to digital (A/D) conversioncircuitry (not shown) for converting analog position signals at theinput 544 into digital signal representations thereof. The I/O interface608 further includes outputs 538, 542 and 540 for producing the fluidcontrol signal, the braking control signal and the vacuum controlsignal. Each of the outputs 538, 542 and 540 may include digital toanalog (D/A) conversion circuitry (not shown) for converting digitalsignals received from the processor into analog signals, suitable forcontrolling the various actuators coupled to the outputs. The processorcircuit 536 may include a plurality of inputs 544 and a plurality ofoutputs 538, 542 and 540 for controlling the plurality of respectivepressure wave generators 104.

While the controller may be conveniently implemented using the processorcircuit 536, the controller may also be implemented using customdesigned analog circuitry or a mixture of analog and digital circuitry.

The operation of the fusion reactor 100 will now be explained withreference to FIG. 3. The recirculation system 130 establishes a flow ofthe liquid medium 120 between the inlet aperture 124 and the outletaperture 128. The recirculation system also ensures that the liquidmedium 120 is at a desired temperature. Fusionable material 138 isintroduced from the reservoir 136 into the recirculating liquid medium120 in the conduit 125 and caused to travel into the inner cavity 122through the inlet aperture 124. The fusionable material target 140,comprising a quantity of the fusionable material 138, is transportedupwards in the inner cavity 122 by its buoyancy and the flow of theliquid medium 120. While the target 140 is moving towards the center ofthe inner cavity 122, the locating system in the controller 142, isoperable to determine a location of the target 140 in response tosignals representing the location of the target produced by the positionsensors 152. When the location system detects that the target 140 isproximate to the center of the inner cavity 122, the controller 142initiates the firing of the pressure wave generators 104. A desiredimpact timing and desired kinetic energy is selected for each pressurewave generator 104 such that the contributions of energy generated byeach pressure wave generator will cause a pressure wave to be generatedin the liquid medium 120 that converges to the determined location ofthe target 140.

In the embodiment shown in FIG. 3, each pressure wave generatorgenerates its contribution to the pressure wave by impacting thetransducer 412. The transducer 412 is moveable and receives energy fromthe piston in the form of kinetic energy and converts that energy into apressure wave in the liquid medium 120. The pressure wave envelopes andconverges on the fusionable material target 140. If the pressure wavehas sufficient amplitude and is sufficiently symmetrically focused onthe target 140 when it reaches the target, the fusionable material willbe compressed to an extent sufficient to increase the pressure andtemperature of the fusionable material 138 contained therein to a levelwhere nuclear fusion reactions are initiated.

Advantageously, the flow of liquid medium 120 between inlet aperture 124and the outlet aperture 128 quickly transports the next fusionablematerial target to the center of the inner cavity 122. Each firing ofthe pressure wave generators 104 initiates fusion reactions, which inturn generates heat. The heat may be extracted by the recirculationsystem 130 and used to generate electrical power.

In other embodiments the transducer may comprise a portion of the wall102 and the piston 410 may impact the portion of the wall 102 directly,thus transferring the kinetic energy of the piston 410 to the exteriorof the wall. The impact causes a compression wave in the wall portionand also elastically displaces the portion of the wall causing thekinetic energy to be converted into a pressure wave at the interior ofthe wall, where the wall is coupled to the liquid medium 120 due to itscontact therewith.

In the operation of the fusion reactor 100, it is desirable that thepressure wave symmetrically converge on target 140 from all sides. Anyasymmetry in the pressure wave may allow the target 140 to distort,which may result in a corresponding decrease of the maximum temperatureand pressure achieved. Consequently, it may be important that theoperation of the pressure wave generators 104 be precisely controlled,which may involve synchronising the firing of the pressure wavegenerators 104. Alternatively, the determined location of the target 140may be utilized to control the firing of the pressure wave generators104, such that the pressure wave converges to the location of the target140, which may not be exactly at the center of the inner cavity 122. Thedesired impact timing and desired amount of kinetic energy for eachpiston may also be selected in order to account for minor mechanicaldifferences between the pressure wave generators 104.

The operation of the pressure wave generators 104 will now be describedin greater detail with reference to FIGS. 5, 6, 7 and 8. In preparationfor firing the pressure wave generators 104, the compressor 446 and thevacuum pump 430 are activated. The compressor 446 pressurises thereservoir 444, which in turn provides actuating energy for the pressurewave generators 104. The reservoir 444 may permit storage of sufficientenergy, in the form of compressed fluid, to actuate a plurality of thepressure wave generators 104 simultaneously or to actuate one or morepressure wave generators several times in succession.

FIGS. 7 and 8 include representations of blocks of codes, encoded in theprogram memory 606 for directing the processor 602 to execute a methodin accordance with one aspect of the invention. The blocks of codes maybe read into the program memory 606 from a CD ROM 612 readable by themedia reader 610. Alternatively the blocks of codes may be provided tothe program memory 606 through an encoded signal.

Referring to FIG. 7, the blocks of codes shown generally at 700 directthe processor to execute an algorithm for initializing the pressure wavegenerator 104 for a firing operation. Block 702 directs the processor602 to read the position signal at the input 544 to check that thepiston 410 is located at an initial position. The initial position maybe abutting the brake 434 but in practice the initial position of thepiston 410 may also be a position partway down the bore 418 of thehousing 400. Advantageously, different pressure wave generators 104 mayhave different initial piston positions, which may be used to accountfor variations between pressure wave generators or to produce differentdesired impact kinetic energy for the pressure wave generators.

Block 704 directs the processor 602 to cause the braking control signalto be produced at the output 542 to cause the brake 434 to apply aholding force to the control rod 422. Block 706 directs the processor602 to cause the fluid control signal to be produced at the output 538to cause the regulator 442 to apply a desired fluid pressure in thecavity 546 behind the piston 410, thus exerting a motive force on thepiston. Block 708 directs the processor 602 to cause the vacuum controlsignal to be produced at the output 540 to cause the vacuum controlvalve 432 to be opened, allowing the vacuum pump 430 to at leastpartially evacuate a cavity 548 in front of the piston 410, so that thepiston will not have to displace the air in the cavity 548 as it movesalong the bore 418. The pressure wave generator 104 is now ready to befired and is prevented from moving by the holding force applied by thebrake 434.

As previously indicated, it is advantageous to accurately control thetiming and kinetic energy of each pressure wave contribution from eachpressure wave generator 104. Accordingly, a schedule of positions may beestablished for each pressure wave generator 104. The schedule for eachpressure wave generator 104 may be determined in response to a locationof the fusionable material target 140 in the fusion reactor, such thatwhen all the pressure wave generators are fired the resulting pressurewave in the liquid medium 120 will converge to the location of thefusionable material target. The schedule for each pressure wavegenerator 104 may be stored in the RAM 604 as a table of positionvalues, each successive value in the table representing a desiredposition at a relative time after the firing of the pressure wavegenerator.

Referring now to FIG. 8 a firing process is shown generally at 800,represented by blocks of codes that direct the processor 602 to executean algorithm for controlling the movement of the piston 410 according tothe schedule of positions so that the piston impacts the transducer 412at a desired time and with a desired amount of kinetic energy. Block 802directs the processor 602 to initiate the firing of the pressure, wavegenerator 104 by generating a release signal and changing the brakingcontrol signal at the output 542 of I/O interface 608 in response to therelease signal such that the holding force applied by the brake 434 tothe control rod 422 is at least partially released, thus causing thepiston 410 to be accelerated under fluid pressure towards the transducer412. The release signal does not cause the braking control signal tocompletely release the brake 434, since it is necessary to apply somerestraining force to the piston 410 to facilitate control of the pistonmovement along the bore 418. The processor 602 thus causes a restrainingsignal to be produced that in turn causes the braking control signal tocause the brake 434 to apply some restraining force to the control rod422. The processor 602 is then able to control the velocity of thepiston 410 by altering the restraining signal thus causing the brakingcontrol signal to be changed, which in turn causes the restraining forceapplied by the brake 434 to be either reduced to allow the piston 410 tospeed up, or increased to cause the piston to slow down.

Conveniently, in this embodiment, the brake 434 may be used to applyboth the holding force and the restraining force. The braking controlsignal produced at the output 542 of the I/O Interface 608 is acombination of the release signal and the restraining signal. Therelease signal and restraining signal may be digital signals havingnumeric signal values. Similarly the braking control signal may also bea digital signal, allowing the braking control signal to be derived froma simple summation of the release signal values and the restrainingsignal values. The I/O interface 608 may subsequently convert thedigital braking control signal into an analog braking control signal forcontrolling the brake 434.

In other embodiments, the holding force may be applied by a separatebrake or other holding force generator in which case the release signalmay be used to control the separate brake.

Once the holding force is released the piston 410 accelerates due to thefluid pressure in the cavity 546 and the control rod 422 moves alongwith the piston 410, causing the marks 424 on the control rod to movepast the position sensor 450, thus causing a time varying analogposition signal to be generated by the position sensor. Block 804directs the processor 602 to receive the analog position signal at theinput 544 and to convert the analog signal input into a plurality ofdigital values, p(t), representing successive positions of the controlrod 422 (and hence the piston 410) in real time. Block 806 directs theprocessor 602 to store the digital values in the RAM 604.

Block 808 directs the processor 602 to compare a value p(t),representing the present position of the piston 410 with a value p(t-1)stored in RAM 604, representing a previous position of the piston. Ifp(t) is greater that p(t-1) then the piston is still moving toward thetransducer 412 and the codes in block 810 direct the processor 602 tocompare p(t) against a desired piston position value p(T), from theschedule of positions, to establish a position error value e(t). If theposition error value e(t) is negative then the present position of thepiston 410 is ahead of the scheduled position, and block 812 directs theprocessor to generate a restraining signal that will cause anappropriate restraining force to be applied to the control rod 422 inorder to slow down the piston 410. The restraining signal may becalculated using a system transfer function comprising a mathematicalexpression of the relationship between the position error value e(t) andthe appropriate restraining signal that will cause the barking controlsignal to cause the brake 434 to apply the appropriate restraining forceto the piston 410. On the other hand if the position error value e(t) ispositive then the present position of the piston 410 is behind thescheduled position, and block 812 directs the processor to generate arestraining signal that will cause an appropriate restraining force tobe applied to the control rod 422 in order to allow piston 410 to speedup.

Block 813 then directs the processor 602 to produce the braking controlsignal at output 542 of the I/O Interface 608 in response to therestraining signal such that an appropriate restraining force is appliedto the control rod 422 by the brake 434. The processor 602 is thendirected back to block 804 for further repetition of the blocks 804 to813. In one embodiment, the piston is accelerated to a velocity of 70meters per second and blocks 804 to 803 are repeated every 10nanoseconds, allowing the impact of the piston 410 to be controlled witha resolution of around 1 micrometer.

If at block 808, p(t) is less than or equal to p(t-1) then the piston410 has impacted the transducer 412 and has either stopped moving orrebounded in the opposite direction. The processor 602 is then directedto block 814 ending execution of the blocks of code 804 to 813.

The block 814 may include codes for implementing an adaptive controlalgorithm, making the system less sensitive to changing environmentalconditions such as temperature, mechanical variances over time of thepressure wave generators 104, and mechanical variances between differentpressure wave generators. Accordingly block 814 may direct the processor602 to modify the transfer function based on a completed operation ofthe pressure wave generator. The transfer function may include amathematical expression having a number of parameters. The parametersmay define various gains of components of the pressure wave generator104 that may vary over time. The processor may use stored values of p(t)and e(t) to calculate a new set of parameters for the transfer function.Advantageously, the adaptive control algorithm may be used to accountfor wearing in of the pistons and other environmental disturbances thatwould be more difficult to account for in a conventional linear controlalgorithm.

Block 702 in FIG. 7 then directs the processor 602 to cause the piston410 to be returned to the initial position in preparation for the nextfiring of the pressure wave generator 104. In one embodiment the fluidpressure may be removed from the fluid port 408 and a fluid pressure maybe applied to the vacuum conduit 428 thus driving the piston back to theinitial position. Block 702 directs the processor 602 to read positionsignals at the input 544 to confirm that the piston 410 is returned tothe initial position.

The operation of the piston 410 and the transducer 412 will now bedescribed with reference to FIG. 20. As previously described, the piston410 is accelerated along the bore 418 by fluid pressure applied to thecavity 546 of the pressure wave generator 104. The piston 410 isdimensioned so that there is a very small gap between the bore 418 andthe piston 410 and when fluid pressure is applied to the cavity 546,some of the fluid leaks through the conduit 2014 and establishes an aircushion between the orifices 2016 and 2018 and the bore 418. Aspreviously indicated, a plurality of such orifices are circumferentiallylocated around the piston 410 and the plurality of air cushions, soestablished, form a cushion of air between the piston 410 and the bore418, thus virtually eliminating friction between the piston and thebore.

The transducer 412 operates by receiving kinetic energy from the piston410 and converting the kinetic energy into a pressure wave in the liquidmedium 120. The outer surface 2030 of the transducer 412 is in contactwith the liquid medium 120 and the pressure exerted by liquid medium 120exerts a force on the transducer 412 that biases the transducer 412 intocontact with the tapered wall portion 2022 prior to impact. When thepiston 410 impacts the transducer 412, the kinetic energy of the pistonat least partially transfers to the transducer 412. The transferredkinetic energy initially accelerates the impact surface 2031 of thetransducer. However, at the instant of impact the outer surface 2030 ofthe transducer 412 is at rest, resulting in the transducer beingelastically compressed by the impact. The impact thus causes acompression wave to propagate through the transducer 412 from the impactsurface 2031 to the outer surface 2030. The outer surface 2030 is alsolater accelerated by the impact which increases the kinetic energy atthe outer surface. The pressure wave coupled into the liquid medium 120thus includes energy from the compression wave in the transducer 412 andkinetic energy due to the displacement of the transducer within the bore2026. The energy in the compression wave couples directly into theliquid medium 120 while the kinetic energy causes a pressure wave to beproduced at the outer surface 2030 by locally compressing the liquidmedium.

At impact, the piston 410 is still under the motive force of the appliedfluid pressure, which together with the kinetic energy of the piston maybe operable to cause the piston to continue to move with the impactsurface 554 of the piston in contact with the impact surface 2031 of thetransducer. The impact also causes a compression wave at an impactsurface 554 of the piston 410, which propagates toward a rear surface555 of the piston. At the rear surface 555, the compression wave isreflected back in the direction of the impact surface 554 but with a 180degrees phase shift i.e. the compression wave becomes an extension wavethus de-compressing the piston material.

Since the transducer 412 and the liquid medium 120 will typically bedifferent materials (transducer 412 may be steel), there may be animpedance mismatch at the outer surface 2030, resulting a reflection ofenergy back towards the impact surface 2031 of the transducer. If thisreflection is sufficiently large the piston 410 may be caused to reboundagainst the applied fluid pressure. In one embodiment it is desirable tominimise any reflection of energy at the outer surface 2030 in order tominimise the rebound of the piston 410. Methods and apparatus forreducing the impedance mismatch are described below.

Once the kinetic energy has been coupled into the pressure wave in theliquid medium 120, the pressure wave travels toward the target 140 andinitiates fusion in the fusionable material 138. Some of the energy thatis not dissipated in initiating the fusion reaction will continue topropagate across the inner cavity 122 to the wall 102 of the fusionreactor 100. Additionally, for a fusionable material 138 ofdeuterium-tritium (D-T), approximately 20% of the fusion energy will bereleased in the form of fast alpha particles. These alpha particles havea very short range in the liquid medium 120 and will therefore deposittheir energy in a very small volume near the location of the target 140.The alpha particle energy produces a further pressure wave that isdirected outwardly towards the wall 102. When these pressure waves reachthe transducer 412 they generate a restoring force returning thetransducer back to its initial position. Some of the energy may alsocouple into the transducer as a compression wave, which may cause thepiston 410 to rebound rearwardly. Advantageously the rebound may be usedto at least partially return the piston 410 to its initial position,while the fluid pressure is still applied, thus conserving the energyrequired to pressurise the fluid.

Referring to FIG. 21, an embodiment of a pressure wave generator forimproving an impedance match between the piston 410 and the liquidmedium 120 is shown generally at 2100. The pressure wave generator 2100includes a housing 2102 that accommodates a transducer 2104 and thepiston 410. The transducer 2104, which in this embodiment does not havea conically tapered shape, further includes a layer of steel 2106, alayer of titanium 2108 and a layer of aluminium 2110. The layers 2106,2108, and 2110 are fused or otherwise secured to each other to form thetransducer 2104.

The acoustic impedance, Z, of a material is defined as:Z=ρ·V   Equation 1where ρ is the density of the medium through which the pressure wavetravels and V is the acoustic velocity of that material. The fraction ofreflected energy for normal incidence at an interface between twodifferent materials is given by $\begin{matrix}{R = \left\lbrack \frac{Z_{2} - Z_{1}}{Z_{2} + Z_{1}} \right\rbrack^{2}} & {{Equation}\quad 2}\end{matrix}$where R is the fraction of reflected energy at an interface between afirst material having an acoustic impedance Z₁, and a second materialhaving an acoustic impedance Z₂. Values of ρ, V and Z for some commonmaterials are listed in Table 1.

Clearly, from Equation 2, when Z₁ and Z₂ are equal, no energy isreflected at the interface but when Z₁ and Z₂ are different, somefraction of the energy is reflected at the interface. TABLE 1 AcousticAcoustic Density Velocity Impedance Material [kg · m⁻³/1000] [km · s⁻¹][10⁶ Rayls] Steel 7.9 5.2 41 Lead 11.3 1.2 12 Titanium 4.5 5.0 22.5Aluminum 2.7 5.2 14

In the case where the liquid medium is molten lead, a direct steel-leadinterface (i.e. the transducer 412 is made from steel and no conicaltaper), the impedance mismatch results in about 30% of the energy beingreflected back from the interface between the transducer 412 and theliquid medium 120 (calculated using Equation 2 and the values in Table1).

In operation the compression wave through the transducer 2104 propagatesthrough the layer 2106 to an interface between the layers 2106 and 2108which is a steel-titanium interface. The reflection at this interfacemay be calculated to be approximately 8.5% using equation 2. Thecompression wave, now diminished by 8.5%, continues to propagate to theinterface between the layers 2108 and 2110 which is a titanium-aluminiuminterface. The reflection at this interface is a further 5.4% or 5% ofthe initial compression wave. The compression wave then propagates tdthe interface between layer 2110 and the liquid medium 120, which is analuminium-lead interface. In this case the reflection is a further 1% or0.6% of the initial compression wave. The total reflection is thusreduces to around 14% (8.5%+5%+0.6%), showing that by choosing asuitable material composition of the transducer 2104 the energyreflection may be substantially reduced. In practice, a variety ofmaterials may be used for the layers for impedance matching thetransducer and the liquid medium.

Referring to FIG. 22, further improvement in the impedance match may beobtained by using a tapered transducer 2202. The transducer 2202includes a first surface 550, a second surface 552 and a conical taper558 between the first and the second surface such that the secondsurface has a greater area than the first surface. The taper 558increases the mass of lead that the transducer 2202 interacts with atthe second surface 552 compared to an un-tapered transducer that hasequal area at each of the first and the second surfaces. For example,for a steel transducer having a smaller diameter of 10 cm and a 1.5 cmtaper, the piston area of the first surface 550 is 79 cm² while the areaof the second surface 552 is 133 cm², resulting is a 70% greater area incontact with the liquid medium 120. The reduction in reflected energymay be estimated by considering that there is an apparent increase inthe density of the liquid medium 120 approximately 1.7 times, whichresults in a reduction in the reflection from the 30% to approximately11% for the case of a steel to lead interface.

In practice, a combination of taper 558 and different material layersmay be employed to achieve the best overall impedance match between thetransducer 412 and the liquid medium 120.

Referring again to FIG. 22, another embodiment of a pressure wavegenerator further includes a conformal disk 2206, attached to thetransducer 2202. The conformal disk 2206 may include copper, a softaluminium alloy, or a synthetic material, or a composite of two or morematerials.

Referring to FIG. 5, if the impact surface 554 of the piston 410 and thetransducer 412 are not parallel, the impact stresses may be concentratedover less then the full surface of the piston and the transducer. Thestress concentration may result in local deformation or wear of theimpact surface 554, which may represent a risk for early failure of thepressure wave generator 104.

Referring again to FIG. 22, in operation the impact surface 554 of thepiston 410 impacts the conformal disk 2206, which is able to deform totake up any misalignment between the piston and the transducer 2202. Inservice, the conformal disk 2206 may become so deformed as to looseeffectiveness for its intended purpose. Advantageously, the conformaldisks 2206 may be may be removable, thus allowing replacement after theconformal disk nears an end of its useful service period.

Alternatively, a layer of conformal metal such as copper may beelectroplated onto the impact surface 554 of the piston 410, or on thefirst surface 550 of the transducer 2202.

The use of the piston 410 together with a moveable transducer 412 hasseveral advantages over the use of a piston that directly impacts thewall 102 of the fusion reactor 100. A first advantage is that directimpact with the wall 102 of the reactor 100 may introduce high stressesin the wall at the location of the impact, making it necessary to eitherlimit the kinetic energy of the impact or to design the wall towithstand such impact stresses. The use of the moveable transducer 412mitigates the problem of stresses in the wall of the vessel.

A second advantage is gained in ease of alignment of the pressure wavegenerators 104. As previously indicated, it is very important that thepressure wave converge symmetrically on the fusionable material 138,which means that the fusion reactor 100 may need to be constructed totight tolerances. The tolerances may be relaxed for a fusion reactor 100that uses the piston 410 together with the transducer 412 since thepressure wave in the liquid medium 120 is originated at the outersurface 2030 of the transducer and the transducer may be aimed byaligning the pressure wave generator 104. As previously indicated, thelongitudinal position and aim of the pressure wave generator 210 may beadjusted using shims 2012 or other adjustment mechanism. Since thefusion reactor 100 may be several meters in diameter, or larger, withthe wall 102, being around 15 cm thick, an opportunity to reduce themanufacturing tolerances thereof may represent a potential costreduction. Advantageously, the transducers 412 facilitate the generationof a pressure wave that symmetrically envelopes and converges on thefusionable material target 140.

Referring to FIG. 3 in one embodiment the fusion reactor 100 includes analignment system for generating alignment information regarding thepressure wave generators 104. The alignment system utilizes theplurality of ultrasonic transceivers 152, mounted on the wall 102between the pressure wave generators 104. Sub groups of the plurality ofultrasonic transceivers may be arranged to form a phased array oftransceivers, which form the basis of a sonography system, similar tosonographic imaging systems used in medical diagnostic imaging.

In operation of the alignment system, individual transceivers 152 in thephased array are excited by signal pulses at the same frequency, but atdifferent phase angles. This results in an ultrasonic beam being focusedon an inside surface of the wall 102 of the fusion reactor 100. Thefrequency and phase of the signal pulses is selected to focus the beamtoward a particular opening 302 accommodating a particular pressure wavegenerator 104 and transducer 412. The surface of the transducer 412reflects the beam back to one of the transceivers 152 which is switchedinto a receiving mode. The transceiver in the receiving mode convertsthe received reflection into a signal waveform that is analysed todetermine the elapsed time between transmitting the signal pulse andreceiving a reflected pulse. The distance between the phase array andsurface of the transducer 412 may then be calculated from the elapsedtime and the speed of sound in the liquid medium 120. Since symmetry ismore important than the exact dimensions in the fusion reactor, it isnot important to have an accurate knowledge of the speed of sound in theliquid medium 120, as long as the environmental conditions do not changesufficiently over the course of the alignment, thus affecting theresults.

By selecting other sub groups in the plurality of transceivers 152,and/or by altering the phase of the signal pulses, a plurality ofmeasurements may be made of all areas of the inside of the fusionreactor 100. The plurality of measurements may be analysed to provide amap of the inside surface of the fusion reactor 100, allowing themeasurement of the relative alignment of each pressure wave generator104 with respect to other pressure wave generators. If necessary, aparticular pressure wave generator 104 may be shimmed or otherwiseadjusted to correct any detected misalignment.

Alternatively, the transceivers 152 may be operated in a different modewhere a single divergent beam is transmitted from one transceiver, and areturned reflection is received by an array of transceivers. Byexamining the elapsed time and the relative phase of the signalsgenerated by the array of transceivers in response to the reflectedbeam, a mapping of the interior surface of fusion reactor 200 may beperformed.

In some embodiments the piston 410 may be used without the transducer412 while still employing the control features described herein (thebrake 434, the position sensor 450, the control rod 422, and thecontroller 142). In such an embodiment the piston 410 may be disposed todirectly strike the wall 102 of the fusion reactor 100. Similarly, inother embodiments the piston 410 and the transducer 412 may be usedwithout implementing all of the control features described herein.

Referring to FIG. 18, a system for fabricating the control rod 422 isshown generally at 1800. In this embodiment the control rod 422 isfabricated from a steel tube 1804, but it may also be fabricated from asolid steel rod. Advantageously the steel tube 1804 may be a commonlyavailable hardened steel shaft, which are available in a range of sizesand have good roundness and surface finish. The control rod may have acircular cross section. The achievable position sensing resolution willalso depend on the size and spacing of the marks 424, but the marksshould not interfere with the operation of the brake 434. A convenientsize for the marks 424, which should provide sufficient positionresolution, would be 10 μm marks at a 10 μm spacing.

The steel tube 1804 is placed on a mandrel 1806 and mounted in a lathetype machine (not shown), that is capable of rotating the steel tube1804. The surface 1802 is first coated with a photoresist solution.Advantageously the photoresist may be sprayed onto the surface 1802while the steel tube 1804 is being rotated in the lathe machine. Thephotoresist coated surface 1802 is then exposed to imaging radiationusing an exposure source 1808. A suitable exposure source is theSQUAREspot® Thermal Imaging head manufactured by Creo Inc, of BurnabyBritish Columbia. The SQUAREspot® Thermal Imaging head provides a laserpower of 20 Watts or more in a plurality of controllable imaging beams,each beam having a diameter of around 8 μm.

The exposure source 1808 is moveable in a transverse direction shown byarrow 1812 and further includes a controller 1810. The controller 1810includes data and control lines 1814 for providing data defining adesired pattern and for controlling the exposure source 1808. Thecontroller 1810 includes circuitry for generating image data defining apattern of indicia to be formed on the control rod 422 and also controlsthe lathe machine rotation and the transverse movement of the exposuresource 1808. The exposure source 1808 is directed by the controller 1810to image a plurality of bands 1816 corresponding to the desired size andspacing of marks 424. The imaging beams selectively pattern thephotoresist layer, hardening exposed areas of photoresist while leavingunexposed areas unchanged.

The steel tube 1804 is then removed from the lathe and placed in asuitable etch solution. The etch solution only attacks the unexposedareas while the hardened areas of resist protect the underlying surface1904. Referring now to FIG. 19, a portion of an etched steel tube isshown in greater detail at 1900. The areas 1904 are protected from theetch solution by the hardened photoresist layer 1906, while the areas1902 are unprotected and are thus attacked by the etch solution. Theetched areas 1902 have a rough texture, due to the activity of the etchsolution, and also have a slight radial offset from the surface 1904 ofthe steel tube 1900. In a subsequent step these areas 1902 may bechemically blackened to further reduce their reflectivity. Thephotoresist layer 1906 may then be removed, leaving a smooth, un-etchedsurface 1904 exposed. The marks 424 are defined by a plurality ofalternating reflective smooth surface areas 1904 and less reflectiverough areas 1902. The smooth reflective areas 1904 provide a suitablesurface for applying a braking force, while the areas 1902, which areslightly recessed from the surface 1904, do not interfere with theoperation of the brake.

While specific embodiments of the invention have been described andillustrated, such embodiments should be considered illustrative of theinvention only and not as limiting the invention as construed inaccordance with the accompanying claims.

1. A method of operating a pressure wave generator in a system ofpressure wave generators for generating a pressure wave in a liquidmedium contained in a fusion reactor, wherein each pressure wavegenerator has a moveable piston and a control rod coupled thereto, themethod comprising: causing the piston to be accelerated toward atransducer coupled to the liquid medium, by applying a motive force tothe piston; applying a restraining force to the control rod to cause thepiston to impact said transducer at a desired time and with a desiredkinetic energy such that said kinetic energy is converted into apressure wave in the liquid medium.
 2. The method of claim 1 whereinapplying said motive force comprises applying a fluid pressure to thepiston.
 3. The method of claim 2 wherein causing the piston to beaccelerated comprises applying a holding force to the control rodoperable to hold the piston stationary while applying a fluid pressureto the piston.
 4. The method of claim 3 comprising using a brake toapply said holding force.
 5. The method of claim 2 wherein causing thepiston to be accelerated comprises releasing a latch coupled to at leastone of the control rod and the piston, said latch being operable to holdthe piston stationary while applying a fluid pressure to the piston. 6.The method of claim 1 further comprising generating a position signalrepresenting a position of the piston and applying said restrainingforce in response to said position signal.
 7. The method of claim 6wherein generating said position signal comprises generating a signalrepresenting a position of the control rod.
 8. The method of claim 6wherein applying said restraining force comprises applying saidrestraining force in response to differences between positions of thepiston and desired piston positions from a schedule of positionsrepresenting a desired piston position relative to time.
 9. The methodof claim 8 wherein applying said restraining force comprises increasingsaid restraining force when a position of the piston is ahead of ascheduled position and decreasing said restraining force when saidposition of the piston is behind said scheduled position.
 10. The methodof claim 8 wherein applying said restraining force comprises producing arestraining force in response to applying a transfer function to atleast one of said differences.
 11. The method of claim 10 furthercomprising modifying said transfer function in response to at least oneof said differences, such that respective differences in a subsequentoperation of the piston are minimized.
 12. A pressure wave generatorapparatus for use in a system of pressure wave generators for generatinga pressure wave in a liquid medium contained in a fusion reactor, theapparatus comprising: a moveable piston; a control rod coupled to saidpiston; a transducer coupled to the liquid medium; means for causingsaid piston to be accelerated toward said transducer, by causing amotive force to be applied to said piston; means for causing arestraining force to be applied said control rod to cause said piston toimpact said transducer at a desired time and with a desired kineticenergy such that said kinetic energy is converted into a pressure wavein the liquid medium.
 13. The apparatus of claim 12 wherein said meansfor causing said motive force to be applied comprises means for applyinga fluid pressure to said piston.
 14. The apparatus of claim 13 furthercomprising means for causing a holding force to be applied to saidcontrol rod, said holding force operable to hold said piston stationarywhile applying a fluid pressure to said piston.
 15. The apparatus ofclaim 12 further comprising means for generating a position signalrepresenting a position of said piston.
 16. The apparatus of claim 15wherein said means for causing said restraining force to be applied tosaid control rod is operably configured to cause said restraining forceto be applied in response to said position signal.
 17. The apparatus ofclaim 15 wherein said means for generating said position signalcomprises means for generating a signal representing a position of saidcontrol rod.
 18. The apparatus of claim 12 wherein said means forcausing said piston to be accelerated toward said transducer comprisesmeans for directing said piston toward a wall containing the liquidmedium in the fusion reactor such that said piston impacts said wall andwherein said wall acts as said transducer coupled to the liquid medium,such that said impact of said piston against said wall causes a pressurewave to be generated in the liquid medium.
 19. The apparatus of claim 12further comprising means for guiding said piston toward said transducer.20. The apparatus of claim 19 wherein said means for guiding comprisesmeans for at least partially evacuating air from movement path of saidpiston.
 21. The apparatus of claim 19 wherein said means for guidingsaid piston comprises a housing having an inside bore.
 22. The apparatusof claim 21 further comprising means for generating an air cushionbetween said piston and said inside bore operable to reduce frictionalforces between said piston and said bore.
 23. The apparatus of claim 21wherein said transducer comprises means for impedance matching saidtransducer to the liquid medium.
 24. The apparatus of claim 12 whereinsaid piston comprises a face operable to impact a face of saidtransducer and wherein said transducer comprises means for reducinglocalized impact stresses between said face of said piston and said faceof said transducer.
 25. A pressure wave generator apparatus for use in asystem of pressure wave generators for generating a pressure wave in aliquid medium contained in a fusion reactor, the apparatus comprising: amoveable piston; a control rod coupled to said piston; a transducercoupled to the liquid medium; a motive force generator for causing saidpiston to be accelerated toward said transducer; a brake for causing arestraining force to be applied said control rod to cause said piston toimpact said transducer at a desired time and with a desired kineticenergy such that said kinetic energy is converted into a pressure wavein the liquid medium.
 26. The apparatus of claim 25 wherein said motiveforce generator comprises a housing for guiding said piston, saidhousing defining a first cavity behind said piston, said cavity having afluid port for applying a fluid pressure to said cavity operable toaccelerate said piston toward said transducer.
 27. The apparatus ofclaim 26 wherein said housing defines a second cavity in front of saidpiston and further comprising a vacuum port in said second cavityoperable to facilitate said acceleration of said piston by at leastpartially evacuating said second cavity.
 28. The apparatus of claim 26further comprising a brake for causing a holding force to be applied tosaid control rod, said holding force operable to hold said pistonstationary while applying a fluid pressure to said piston.
 29. Theapparatus of claim 26 further comprising a latch coupled to at least oneof said control rod and said piston, said latch being operablyconfigured to be released while fluid pressure is applied to said pistonto permit said piston to accelerate under said fluid pressure.
 30. Theapparatus of claim 25 further comprising a position sensor forgenerating a position signal representing a position of said piston. 31.The apparatus of claim 30 wherein said control rod has a plurality ofindicia on a surface thereof and said position sensor comprises: anilluminator for directing a beam of light towards the indicia; and aphotodetector for generating a signal representing an intensity of lightreflected from the indicia, such that when said piston is accelerated,movement of said control rod causes said photodetector to generate asignal of varying intensity representing said position of said controlrod.
 32. The apparatus of claim 30 wherein said brake is operablyconfigured to cause said restraining force to be applied in response tosaid position signal.
 33. The apparatus of claim 30 wherein said brakeis operably configured to cause said restraining force to be applied inresponse to differences between positions of said piston and desiredpiston positions from a schedule of positions representing desiredpiston positions relative to time.
 34. The apparatus of claim 33 whereinsaid brake is operably configured to cause said restraining force to beincreased when a position of said piston is ahead of a scheduledposition and to cause said restraining force to be decreased when saidposition of said piston is behind said scheduled position.
 35. Theapparatus of claim 33 wherein said brake is operably configured to causesaid restraining force to be applied in response to applying a transferfunction to said differences.
 36. The apparatus of claim 35 furthercomprising a controller for modifying said transfer function in responseto said differences, such that respective differences in a subsequentoperation of said piston are minimized.
 37. The apparatus of claim 25wherein said motive force generator is operably configured to directsaid piston toward a wall containing the liquid medium in the fusionreactor such that said piston impacts said wall and wherein said wallacts as said transducer coupled to the liquid medium, such that saidimpact of said piston against said wall causes a pressure wave to begenerated in the liquid medium.
 38. The apparatus of claim 25 whereinsaid transducer comprises a member mounted on a wall containing theliquid medium in the fusion reactor and wherein the pressure wavegenerator apparatus is coupled to said wall such that said piston isdisposed to impact said member.
 39. The apparatus of claim 25 comprisinga housing for guiding said moveable piston, said housing having anoutside surface and an inside bore.
 40. The apparatus of claim 39wherein said outside surface is operable to fit complementarily into anopening in a wall containing the liquid medium in the fusion reactor.41. The apparatus of claim 39 wherein said piston comprises a pluralityof fluid orifices disposed between said piston and said inside bore ofsaid housing, said orifices being operably configured receivepressurized fluid and to generate an air cushion between said piston andsaid inside bore for reducing frictional forces between said piston andsaid bore.
 42. The apparatus of claim 25 wherein said brake comprises:an actuator; a brake pad operable to generate said restraining force byfrictionally engaging a surface of said control rod in response to anactuation force applied by said actuator.
 43. The apparatus of claim 42wherein said actuator comprises a piezoelectric material.
 44. Theapparatus of claim 25 wherein said brake further comprises a magneticcircuit operably configured to establish a magnetic field through saidcontrol rod thereby generating eddy currents in said control rod whensaid control rod moves with respect to said magnetic circuit, saidgeneration of said eddy currents operable to apply said restrainingforce to said control rod.
 45. The apparatus of claim 25 wherein saidbrake comprises: a magnetic fluid in contact with said control rod; amagnetic circuit operably configured to generate said restraining forceby causing a magnetic field to be coupled through said magnetic fluidand said control rod.
 46. A method of generating a pressure wave foractivating a fusion reaction in fusionable material in a liquid medium,the method comprising: causing pistons of respective ones of a pluralityof pressure wave generators to be accelerated toward respectivetransducers coupled to the liquid medium, by applying respective motiveforces to said pistons; causing restraining forces to be applied torespective control rods connected to respective pistons to cause saidrespective pistons to impact said transducer at respective desired timesand with respective desired amounts of kinetic energy such that saidrespective desired amounts of kinetic energy are converted into apressure wave that converges toward the fusionable material in theliquid medium.
 47. The method of claim 46 further comprising introducingfusionable material into the liquid medium.
 48. The method of claim 46further comprising locating said fusionable material in the liquidmedium.
 49. The method of claim 48 further comprising determining saiddesired times and said desired amounts of kinetic energy in response toa location of said fusionable material.
 50. The method of claim 48further comprising producing location signals representing a location ofsaid fusionable material in the liquid medium.
 51. The method of claim50 further comprising producing release signals for causing said pistonsto be accelerated and producing restraining signals for causing saidrestraining force to be applied to said control rods in response to saidlocation signals.
 52. The method of claim 51 further comprisingreceiving said release signals at actuators and causing said actuatorsto release said pistons for movement in response to said releasesignals.
 53. The method of claim 51 further comprising receiving saidrestraining signals at brakes and causing said brakes to apply saidrestraining forces to said control rods in response to said restrainingsignals.
 54. The method of claim 51 wherein at least one of said desiredtimes and desired kinetic energies is determined in response to saidlocation signals.
 55. The method of claim 51 further comprisinggenerating position signals representing positions of respective pistonsand causing said restraining forces to be applied in response to saidposition signals and said location signals.
 56. A computer readablemedium encoded with codes for directing a processor circuit to carry outthe method of claim
 46. 57. A computer readable signal encoded withcodes for directing a processor circuit to carry out the method of claim46.
 58. An apparatus for generating a pressure wave for activating afusion reaction in fusionable material in a liquid medium, the apparatuscomprising: a plurality of pressure wave generators having respectivemoveable pistons, said pistons having respective control rods connectedthereto; a plurality of transducers coupled to the liquid medium; meansfor causing said pistons of respective ones of said plurality of saidpressure wave generators to be accelerated toward respective ones ofsaid plurality of transducers; means for causing restraining forces tobe applied to respective control rods to cause respective pistons toimpact respective transducers at respective desired times and withrespective desired amounts of kinetic energy such that said respectivedesired amounts of kinetic energy are converted into a pressure wavethat converges toward said fusionable material in the liquid medium. 59.The apparatus of claim 58 further comprising means for causingfusionable material to be introduced into the liquid medium.
 60. Theapparatus of claim 58 further comprising means for locating saidfusionable material in the liquid medium.
 61. The apparatus of claim 60further comprising means for determining said desired times and saiddesired amounts of kinetic energy in response to a location of saidfusionable material.
 62. The apparatus of claim 58 further comprisingmeans for producing location signals representing a location of saidfusionable material in the liquid medium.
 63. The apparatus of claim 62wherein said means for causing said pistons to be accelerated comprisesmotive force generating means for generating forces on said pistons inrespective directions of desired movement of said respective pistons andholding means for holding said piston stationary while said forces areapplied.
 64. The apparatus of claim 63 further comprising means forproducing release signals operable to be received by said holding meansand said holding means being responsive to said release signals torelease said pistons to cause said pistons to be accelerated in responseto said forces generated by said motive force generating means andfurther comprising means for producing restraining signals to cause saidrestraining force to be applied to said control rods in response to saidlocation signals.
 65. The apparatus of claim 63 further comprising meansfor generating position signals representing positions of respectivepistons to cause said restraining forces to be applied in response tosaid position signals and said location signals.
 66. The apparatus ofclaim 65 wherein said means for causing restraining forces to be appliedis operably configured to determine at least one of said desired timesand desired kinetic energies in response to said location signals. 67.An apparatus for generating a pressure wave for activating a fusionreaction in fusionable material in a liquid medium, the apparatuscomprising: a plurality of pressure wave generators having respectivemoveable pistons, said pistons having respective control rods connectedthereto; a plurality of transducers coupled to the liquid medium; aplurality of motive force generators for causing said pistons ofrespective ones of said plurality of said pressure wave generators to beaccelerated toward respective ones of said plurality of transducers; aplurality of brakes for causing restraining forces to be applied torespective control rods to cause respective pistons to impact respectivetransducers at respective desired times and with respective desiredamounts of kinetic energy such that said respective desired amounts ofkinetic energy are converted into a pressure wave that converges towardsaid fusionable material in the liquid medium.
 68. The apparatus ofclaim 67 further comprising an aperture for causing fusionable materialto be introduced into the liquid medium.
 69. The apparatus of claim 67further comprising a fusionable material locating system operable tolocate said fusionable material in the liquid medium.
 70. The apparatusof claim 69 further comprising a controller for determining said desiredtimes and said desired amounts of kinetic energy in response to alocation of said fusionable material.
 71. The apparatus of claim 67further comprising location sensors for producing location signalsrepresenting a location of the fusionable material in the liquid medium.72. The apparatus of claim 71 wherein said motive force generators areoperably configured to generate forces on said pistons in respectivedirections of desired movement of said respective pistons and furthercomprising a brake for holding said piston stationary while said forcesare applied.
 73. The apparatus of claim 72 further comprising acontroller for producing release signals operable to be received by saidbrakes, said brakes being responsive to said release signals to releasesaid pistons to cause said pistons to be accelerated in response to saidforces generated by said motive force generators said brakes operableconfigured for producing restraining signals to cause said restrainingforce to be applied to said control rods in response to said locationsignals.
 74. The apparatus of claim 72 further comprising positionsensors for generating position signals representing positions ofrespective pistons to cause said restraining forces to be applied inresponse to said position signals and said location signals.
 75. Theapparatus of claim 73 wherein said controller for causing restrainingforces to be applied is operably configured to determine at least one ofsaid desired times and desired kinetic energies in response to saidlocation signals.
 76. The apparatus of claim 67 further comprising aplurality of location sensors operably configured to produce ultrasonicbeams and to receive reflections of said ultrasonic beams, saidreflections of said ultrasonic beams representing an alignment ofrespective ones of said pressure wave generators.
 77. A method ofoperating a pressure wave generator in a system of pressure wavegenerators for generating a pressure wave in a liquid medium containedin a fusion reactor, the method comprising: causing a moving pistonhaving kinetic energy to impact a moveable transducer coupled to theliquid medium; converting at least a portion of said kinetic energy intoa pressure wave in the liquid medium such that said pressure waveenvelopes and converges on a fusionable material in the liquid medium.78. A pressure wave generator apparatus for use in a system of pressurewave generators for generating a pressure wave in a liquid mediumcontained in a fusion reactor, the apparatus comprising: a moveablepiston; a moveable transducer coupled to the liquid medium; means forcausing said piston having kinetic energy to impact said transducer;means for converting at least a portion of said kinetic energy into apressure wave in the liquid medium, said pressure wave operable toenvelope and converge on a fusionable material in the liquid medium. 79.The apparatus of claim 78 further comprising means for guiding saidpiston toward said transducer.
 80. The apparatus of claim 78 whereinsaid transducer comprises means for impedance matching said transducerto the liquid medium.
 81. The apparatus of claim 78 wherein said pistonhas a face operable to impact a face of said transducer and wherein saidtransducer comprises means for reducing localized impact stressesbetween said face of said piston and said face of said transducer.
 82. Apressure wave generator apparatus for use in a system of pressure wavegenerators for generating a pressure wave in a liquid medium containedin a fusion reactor, the apparatus comprising: a moveable piston; amoveable transducer coupled to the liquid medium; a motive forcegenerator for causing said piston having kinetic energy to impact saidtransducer such that at least a portion of said kinetic energy isconverted into a pressure wave in the liquid medium, said pressure wavebeing formed such that it envelopes and converges on a fusionablematerial in the liquid medium.
 83. The apparatus of claim 82 whereinsaid transducer comprises a plurality of layers of materials havingtransmission properties, each material having different transmissionproperties, said materials being selected and arranged in said layerssuch that said transducer is generally impedance matched to the liquidmedium.
 84. The apparatus of claim 82 wherein said transducer comprisesa member mounted on a wall containing the liquid medium in the fusionreactor and wherein the pressure wave generator apparatus is coupled tosaid wall such that said piston is disposed to impact said member. 85.The apparatus of claim 82 further comprising a housing having an outsidesurface and an inside bore, said inside bore operable to guide saidpiston toward said transducer.
 86. The apparatus of claim 85 whereinsaid outside surface is operable to fit complementarily into an openingin a wall containing the liquid medium in the fusion reactor.
 87. Theapparatus of claim 85 wherein said housing has a first area defined by afirst wall portion operably configured to hold said transducer in aposition in which it will be impacted by said piston.
 88. The apparatusof claim 87 wherein said transducer comprises a member having an outsidesurface having a first portion defining a shape complementary to saidfirst wall portion.
 89. The apparatus of claim 85 wherein said housinghas a first area defined by a first wall portion having a first insidediameter and wherein said housing has a second area defined by a secondwall portion having a second inside diameter and wherein said housinghas a third area defined by a tapered third wall portion located betweensaid first and second wall portions, said first wall portion beingoperable to guide said piston and said second and third wall portionsbeing operable to hold said transducer.
 90. The apparatus of claim 89wherein said transducer comprises a member having an outside surfacehaving first and second portions defining a shape complementary to saidsecond and third wall portions of said housing.
 91. The apparatus ofclaim 90 wherein said first inside diameter is less than said secondinside diameter.
 92. The apparatus of claim 82 wherein said piston has aface operable to impact a face of said transducer and wherein saidtransducer comprises a conformal member for reducing localized impactstresses between said face of said piston and said face of saidtransducer.